> JSON is agnostic about the semantics of numbers. […] JSON instead offers only the representation of numbers that humans use: a sequence of digits. […] That is enough to allow interchange.
But can you encode/decode an arbitrary integer or a float? Probably not!
* Float values like Infinity or NaN cannot be represented.
* JSON doesn't have separate representation for ints and floats. If an implementation decodes an integer value as a float, this might lose precision.
* JSON doesn't impose any size limits. A JSON number could validly describe a 1000-bit integer, but no reasonable implementation would be able to decode this.
The result is that sane programs – that don't want to be at the mercy of whatever JSON implementation processes the document – might encode large integers as strings. In particular, integers beyond JavaScript's Number.MAX_SAFE_INTEGER (2^53 - 1) should be considered unsafe in a JSON document.
Another result is that no real-world JSON representation can round-trip “correctly”: instead of treating numbers as “a sequence of digits” they might convert them to a float64, in which case a JSON → data model → JSON roundtrip might result in a different document. I would consider that to be a problem due to underspecification.